Techniques for Line Narrowing
Techniques for decreasing the bandwidth of the output of lasers are well known. Several such techniques used on excimer lasers are discussed by John F. Reintjes at pages 44-50 in Laser Handbook, Vol. 5, North-Holland Physics Publishing, Elsevier Science Publishers B. V. These techniques include the utilization of gratings, including echelle gratings for wavelength selection. Use of beam expanding prisms ahead of the grating can increase the effectiveness of the grating.
A prior art narrow band excimer laser is shown in FIG. 1. The resonance cavity of excimer laser 2 is formed by output coupler 4 (which is a partially reflecting mirror) and echelle grating 16. A portion of the laser output beam 20 (having a cross section of about 3 mm in the horizontal direction and 20 mm in the vertical direction) exits the rear of laser chamber 3. This portion of the beam is expanded in the horizonal direction by prisms 8, 10 and 12 and reflected by mirror 14 onto echelle grating 16. Mirror 14 is pivoted to select the narrow band output for laser 2. Grating 16 is arranged in Littrow configuration so that the selected narrow band of wavelengths is reflected back off mirror 14 and back through prisms 12, 10 and 8 and into chamber 3 for amplification. Light at wavelengths outside the selected narrow band is disbursed so that this disbursed out-of-band light is not reflected back into the laser chamber. Total beam expansion for this laser is about 20.times.. The beam has a horizontal polarization (P-polarization for the prisms with entrance surface arranged vertically). Typical KrF lasers operating in a pulse mode may have a cavity length of about 1 m and produce pulses having a duration of about 15 to 25 ns. Thus, photons within the resonance cavity will make, on the average, about 3 to 5 round trips within the cavity. On each round trip, about 90 percent of the beam exits at the output coupler and about 10 percent is sent back for further amplification and line narrowing. The beam is repeatedly line narrowed as it passes through the line narrowing module.
With this prior art arrangement, the bandwidth of the excimer laser is reduced from its natural bandwidth of about 300 pm (full width half maximum or FWHM) to about 0.8 pm for KrF lasers and about 0.6 pm for ArF lasers. Another important measure of the pulse quality is referred to as the "95% integral". This is the spectral width of the portion of the pulse containing 95% of the pulse energy. Prior art KrF laser can provide "95% integral" values of around 3 pm over the lifetime of the laser and prior art ArF lasers can provide 95% integral values of about 1.5 pm.
Some applications of KrF and ArF lasers, however, require greater narrowing of the bandwidth. For example, some micro lithography applications require smaller bandwidths such as FWHM values of 0.5 pm and 0.4 pm for KrF and ArF, respectively, and 95% integral values of 2.0 pm and 1.0 pm, respectively. The output beam of the ArF laser is about 193 nm. Very few optical materials can survive substantial exposure to this energetic light. Optical coatings for this wavelength are expensive and often short lived. The problem is even more severe for beams at shorter wavelengths such as the beam from F.sub.2 lasers.
One prior art line narrowing method is to add an etalon within the resonance cavity. In this case, the etalon is operated in a transmissive mode and the light is additionally line narrowed as it passes through the etalon. In such system one should use a relatively high finesse etalon, with a finesse value f of about 1 or higher which produces sharp fringe patterns. The finesse value f is determined by the equation: EQU f=.pi.r.sup.1/2 /(l-r)
where r is the reflectivity of the etalon surfaces. The dependence of etalon transmission spectrum on r is shown graphically in FIG. 2 which is extracted from page 298 of Fundamentals of Optics by Jenkins and White, published by McGraw Hill. From FIG. 2, it is obvious why prior art transmissive etalons used for line narrowing have surfaces with reflectance of about 50% to 80% (see curves B and C of FIG. 2). FIG. 2 also shows that it would not be practical to use curve A-type low finesse etalon as its line-narrowing efficiency in this prior art arrangement would be very low. Prior art high finesse etalons used with diffraction gratings should enhance the line-narrowing capabilities provided by diffraction grating, which, in general, improves the laser line-width. The major disadvantages of this technique are that the many reflections within the etalon tend to heat up the etalon producing thermal distortions and that the tuning of the etalon synchronously with the grating presents a real technical challenge and is difficult to accomplish in practice.
What is needed is a better technique for line narrowing and tuning of KrF and ArF excimer lasers as well as F.sub.2 lasers.